The artist Andrew Goldsworthy has created a beautiful sculpture called 'Spire' in the Presidio, made of felled trees from the site of the sculpture. The area around the Spire has been replanted with saplings, so I'm really curious to see how the look of the sculpture will change over time as nature takes its course. The Spire is alongside one of my favorite nature walks in San Francisco, so I'm planning to take pictures of it every time I go by (as long as I remember my camera) and post them here.
Here's a clip from the wonderful film about his artwork, Rivers and Tides.